摘要：Objectives. I examined the association between income inequality and population health and tested whether this association was mediated by interpersonal trust or public expenditures on health. Methods. Individual data on trust were collected from 48 641 adults in 33 countries. These data were linked to country data on income inequality, public health expenditures, healthy life expectancy, and adult mortality. Regression analyses tested for statistical mediation of the association between income inequality and population health outcomes by country differences in trust and health expenditures. Results. Income inequality correlated with country differences in trust ( r = −0.51), health expenditures ( r = −0.45), life expectancy ( r = −0.74), and mortality ( r = 0.55). Trust correlated with life expectancy ( r = 0.48) and mortality ( r = −0.47) and partly mediated their relations to income inequality. Health expenditures did not correlate with life expectancy and mortality, and health expenditures did not mediate links between inequality and health. Conclusions. Income inequality might contribute to short life expectancy and adult mortality in part because of societal differences in trust. Societies with low levels of trust may lack the capacity to create the kind of social supports and connections that promote health and successful aging. Social inequalities in health are closely aligned with individual differences in income. At every level of socioeconomic status, health tends to be better on the level above and poorer on the level below, even among those who are not poor and enjoy equal access to health services. 1 , 2 Research also shows that health problems that are associated with socioeconomic status are more common in societies that have wider distributions of personal income. 3 It is well-documented that international differences in income inequality (i.e., size of the gap between rich and poor) are associated with rates of mortality 4 and with various mental and physical health problems. 5 – 8 Opinions are divided regarding the contextual mechanisms that might account for the association between income inequality and health. One line of research focuses on the psychosocial impact of inequality and the breakdown of “social capital,” which is defined as features of social organization—such as networks, norms, and interpersonal trust—that facilitate coordination and cooperation for mutual benefit. 2 , 9 Wilkinson, Kawachi, and others have suggested that large income differences intensify social hierarchies and increase class conflict and feelings of relative deprivation while simultaneously reducing levels of interpersonal trust, social cohesion, and other dimensions of social capital that promote health. 3 , 10 – 12 The alternative “neomaterialist” hypothesis suggests that income inequality inhibits public expenditures on important services and infrastructure that promote health. 13 – 17 In the United States, for instance, state expenditures on public health and education negatively correlate with income inequality and adult mortality. 13 , 16 It remains undetermined whether international differences in public expenditures account for the association between income inequality and health. The neomaterialist and social capital hypotheses are not mutually exclusive. Kawachi and Kennedy observed that US state populations with low levels of trust are also characterized by values that support a minimal role for government in reducing health inequalities. 18 Putnam's index of health and health care in the United States (which included expenditures on health care) was highly correlated with an index of social capital. 9 Therefore, it could be the case that more equal, more trusting societies are also more willing to support government spending on goods and services that advance the common good, compared with less equal, less trusting societies. It is important to understand which factors account for the association between income inequality and population health. A piecemeal evidence base shows inconsistent findings for mediation by psychosocial and neomaterial paths. Inconsistencies among studies with regard to sample selection criteria, tests of mediated effects, and measures of income inequality have made it difficult to weigh the evidence in favor of either hypothesis. 11 , 19 – 21 As a result, previous claims that the relationship between income inequality and poor health is mediated by trust, social capital, or public expenditures 22 —or that the relation is simply a statistical artifact caused by confounding effects of individual income, 23 race, 24 or education 25 —have not all been based on rigorous tests of statistical mediation. One issue in particular muddies the water when testing mediated effects: small changes in a regression slope or correlation coefficient that occur when a third variable is controlled can easily cause the statistic to change from significance ( P < .05) to nonsignificance ( P > .05), even when the third variable does not account for a significant proportion of shared variance. Negligible change from significance to nonsignificance does not, in itself, establish mediation. 26 Kawachi et al. addressed this issue by using path analysis to show significant mediated effects of income inequality (via social capital) on mortality 11 and births to adolescents. 20 However, these studies did not include similar mediation analyses of public expenditures. There has not been a direct comparison of psychosocial and neomaterial paths in accounting for the association between income inequality and health. Therefore, my aim in the current study was to test the association between income inequality and 2 indicators of population health—healthy life expectancy and adult mortality—and then test how much this association was mediated by differences in a proxy indicator of social capital (interpersonal trust) and by public expenditures on health. Of course, trust is just a single aspect of social capital that could mediate links between inequality and health, and expenditures on services other than health might also relate to health. But by using a consistent set of data on income inequality and population health, I explored whether their association (if significant) was better explained by a psychosocial path or a neomaterial path. In disadvantaged populations, healthy life expectancy (also referred to as “disability-free life expectancy”) represents the burden of ill health better than total life expectancy does, according to the World Health Organization, 27 so I used healthy life expectancy as an indicator of population health. Adult mortality was used as a general indicator of population health.
